In the QA debugger there is no way to set the parameter to '' without modifying the parameter after starting to execute it. Typing '' makes the parameter '''', typing nothing will give an error, typing a single space will make the parameter ' '

If you need to debug the proc as it is then you have to set the parameter to whatever you like first and then when you step onto the first line of the proc, change it to be the empty string '' This seems to be a shortcoming of the way the debugger is implemented in qa

You could use the default parameter as a work around, but this does change the proc slightly in that it now has an optional parameter rather than a mandatory one, also be careful since len( null ) is null as far as I know which is not = 0 or <> 0